Teach while loops to not hang forever with no-execute

Fixes #1543

exec.cpp

@@ -592,6 +592,7 @@ void exec_job(parser_t &parser, job_t *j)
     CHECK(j,);
     CHECK_BLOCK();
 
+    /* If fish was invoked with -n or --no-execute, then no_exec will be set and we do nothing. */
     if (no_exec)
     {
         return;

parse_execution.cpp

@@ -673,6 +673,12 @@ parse_execution_result_t parse_execution_context_t::run_while_statement(const pa
                 break;
             }
         }
+        
+        /* no_exec means that fish was invoked with -n or --no-execute. If set, we allow the loop to not-execute once so its contents can be checked, and then break */
+        if (no_exec)
+        {
+            break;
+        }
     }
 
     /* Done */

tests/test1.in

@@ -123,6 +123,10 @@ echo -e Catch your breath
 echo -e 'abc\x21def'
 echo -e 'abc\x211def'
 
+# Make sure while loops don't run forever with no-exec (#1543)
+echo "Checking for infinite loops in no-execute"
+echo "while true; end" | ../fish --no-execute
+
 function always_fails
     if true
         return 1

tests/test1.out

@@ -37,4 +37,5 @@ abc
 Catch your breath
 abc!def
 abc!1def
+Checking for infinite loops in no-execute
 1